(A) FIELD OF THE INVENTION
For manufacturing artificial graphite electrodes for use in ultra-high-powered electric furnaces, anisotropic needle coke that can be readily graphitized is popularly used.
This invention relates to a process for preparing such needle coke. More particularly, it relates to a process for preparing needle coke by removing insoluble substances containing quinoline insoluble materials from coal tar and/or coal tar pitch.
(B) DESCRIPTION OF THE PRIOR ART
Various proposals have recently been made for the preparation of needle coke from such coal tar and/or coal tar pitch as contain quinoline insoluble materials, which have been thought to be unstable for the purpose, by removing undesirable insoluble components. For instance, one of such known processes comprises heat-treating coal tar pitch under a pressure between 0 and 10 kg/cm.sup.2 and at a temperature between 350.degree. and 390.degree. C. Then, quinoline insoluble materials are filtered off through a 5 to 10 .mu.m filter, at a temperature between the softening point of the pitch and 350.degree. C. The resulting filtrate is processed into readily graphitizable needle coke. According to another known process, 100 parts of coal tar pitch is diluted with 50 parts of wash oil, by weight. Quinoline insoluble materials contained in the diluted coal tar pitch are filtered off by a vacuum rotary filter at 140.degree. C. Then, the wash oil is separated from the refined pitch by vacuum distillation. The resulting refined pitch is processed into needle or linear shaped dry-distilled coke. However, these conventional processes involve great difficulties in processing large quantities of materials.
For instance, the separation of insoluble substances from a highly viscous mixture cannot be attained unless a large quantity of a solvent which has a high dissolving power is used. Even then, no great effect is achieved, since quinoline insoluble materials consist of very small grains dispersed or suspended in the pitch. Conversely, difficulty is encountered in disposing of the large quantity of solvent used.
Further, there are many problems that prevent efficient separation, such as filter clogging, difficulties in continuing centrifuge operation and in equipment maintenance due to splashing and adhesion of tacky substances. In addition, provision of a centrifuge and other large equipment leads to disadvantageously high cost. For these reasons, the conventionally proposed processes may be practiced on the laboratory scale, but are not advantageous on the industrial scale.